Jorge:

Typically the date is at the end of the proof data string and hyphenated(?). You can see the "77" at the end of the above date string on the left side of watertable. Also the date appears on the right flat atop the Shield "N" stamp. Some earlier versions didn't have the hyphen and I can't say just how many models there were but they seem to concide with the year of the change.

Kind Regards,

Raimey
rse